uss: Allocate buffer space for trailing NUL

uss: Allocate buffer space for trailing NUL

Appease gcc8's -Wformat-truncation engine.

Reviewed-on: https://gerrit.openafs.org/13467
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Cheyenne Wills <cwills@sinenomine.net>
Reviewed-by: Michael Meffie <mmeffie@sinenomine.net>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
(cherry picked from commit 9a5ba85d1853327d8184287e58a6e03fabaaf23d)

Change-Id: I9900452b951d600e6358b216015fe4fcdf8f633e
Reviewed-on: https://gerrit.openafs.org/13736
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Marcio Brito Barbosa <mbarbosa@sinenomine.net>
Reviewed-by: Michael Meffie <mmeffie@sinenomine.net>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>